The current state-of-the-art for fixed temperature cells employ the temperature of the triple-point-of-water as a defined fixed point common to the International and the Kelvin Temperature scales. This defined fixed point is the temperature of pure water, ice, and water vapor in thermal equilibrium. This temperature has the assigned value of +0.01.degree. C. on the International Practical Temperature Scale of 1948, and the value of 273.16.degree. K. on the Kelvin scale.
Representative state-of-the-art, triple-point-of-water cells are the Jarrett cells which consist of cylindrical borosilicate glass containers with a reentrant tube which serves as a thermometric well. The cells are thoroughly cleaned, filled with high purity gas-free water, and then sealed.
Prior to use, the cells are frozen by employing a manual process with dry ice for refrigerating the inside of the reentrant thermometer well so that the water freezes from the thermometer well outward to form a mantle of ice around the well. Great care and expertise are required to prevent catastrophically rupturing the cell.
A typical triple-point-of-water cell, such as the Jarrett cell when in operation, requires a vertical operational technique; i.e., the complete assembly is mounted vertically. The complete assembly includes an outer container for housing an ice and water mixture for maintaining an inner suspended cell at ice, and water, and water vapor equilibrium temperature. The triple-point-of-water cell consists of a cylindrical borosilicate glass container with a reentrant tube which serves as a thermometer well. The cells are prepared by thoroughly cleaning, filling with high purity gas-free water, and then sealing. The cell is usually provided with a brass or aluminum metal bushing near the bottom of the thermometer well for supporting a thermometer being calibrated or tested in a vertical position so that the bulb is held off the bottom of the thermometer well. Another purpose of the metal bushing is to increase the thermal conduction between the thermometer and the ice-water interface. A well established equilibrium condition is guaranteed to be within +0.000,00.degree. or -0.000,15.degree. C. of the triple point of pure ordinary water.
The conventional typical triple-point-of-water cell requires a central thermowell and the delicate process of generating an ice mantle. These cells are extremely fragile and must be carefully handled and stored to prevent breakage. The vertical technique employed in conventional triple-point-of-water cell results in a tendency, as the ice melts around the thermowell, to cause the ice mantle to come loose and float up and away from the thermometer tip. This tendency of losing the ice mantle from the vicinity of the thermometer tip results in a decrease of the useful time for the conventional cell since the whole equilibrium condition is based on achieving the condition of pure water, ice, and water vapor in thermal equilibrium with the temperature of an assigned value of +0.01.degree. C.
The need for a more practical, rugged, device to replace the delicate, awkward, and expensive to maintain triple-point-of-water cell is evident.
Therefore, an object of this invention is to provide a calibration system having a plurality of individual cells which are mounted together in thermal contact with a heat conducting fixture having a central thermowell for inserting the thermometer which is being calibrated by the calibration system.
Another object of this invention is to provide a calibration system which provides for positioning the bulb of the thermometer at a more horizontal position rather than vertical whereby the bulb of the thermometer is positioned a few degrees above horizontal so that any ice melts tends to float upward in the vicinity of the bulb of the thermometer.
Still, a further object of this invention is to provide an electronic triple point calibration system having freezing means for generating the triple-point-of-water temperature, or alternately after cells are frozen for use, the electronic triple point calibration system is maintained for ready use by an electronic freezing point system which is automated through temperature sensing means, switching means, and refrigerating means for generating or regenerating the triple-point-of-water temperature equilibrium conditions thereby maintaining same equilibrium conditions in a ready for use state.